Uranus

Two views of Uranus, one in true color (left) and
the other in false color, compiled from images returned Jan. 17, 1986,
by Voyager 2. The spacecraft was 9.1 million km (5.7 million miles)
from the planet, several days from closest approach.

The seventh planet from the Sun and the third largest of the gas
giants (narrowly beating out Neptune)
with a diameter about four times that of Earth.

Discovery

Uranus was discovered by William Herschel
in 1781. Others had seen it earlier – at its brightest it is just
visible to the naked eye – but had taken it to be a star because it
doesn't move perceptibly from one night to the next. The earliest recorded
sighting was in 1690 when John Flamsteed
catalogued it as 34 Taurus. William Herschel originally called it Georgium
Sidus (George's Star) in honor of King George III, while French astronomers
began calling it Herschel. It was Johann Bode
who proposed the name Uranus, after the Greek god of the heavens, but this
didn't come into common usage until around 1850.

Strange
spin

Uranus's most extraordinary feature is the tilt of its axis – almost
98° (or 82° if it is taken to be retrograde),
so that the planet effectively spins around on its side. As a result, for
part of its orbit, one pole continually faces the Sun while the other is
in total darkness. Half an orbit later, the roles (and poles) are reversed.
In between, the Sun rises and sets around the equator normally. For Uranus
to be in such a position, it was almost certainly struck a formidable blow
by another massive object. Since the plane containing its 13 rings (see
Uranus, rings) and 27 known moons (see Uranus,
moons) is similarly tilted, this impact presumably took place during
or shortly after the accretionary phase of the planets.

Atmosphere

The atmosphere of Uranus is composed of hydrogen (83%), helium (15%), methane
(2%), and trace amounts of acetylene and other hydrocarbons. Its bluish
hue stems from an upper methane haze that absorbs strongly at red wavelengths
– leaving a featureless blue planet in our telescopes. During Voyager
2's flyby in 1986, Uranus's banded cloud patterns were extremely bland
and faint; more recent Hubble Space Telescope observations, however, have
shown a more strongly banded appearance now that the Sun is getting closer
to being directly overhead at Uranus' equator, a position it will assume
in 2007.

A single image of Uranus taken in 2004 shows 18 distinct cloud systems –
eight more than Voyager saw during its entire months-long flyby. And one
set of images taken with the Keck II telescope in Hawaii in summer 2004
shows an extremely bright cloud reaching up high above the planet's opaque
methane layers in its southern hemisphere.

Until 10 years ago, the only pictures sharp enough to show any cloud features
were those from Voyager. But then images from the Hubble Space Telescope
began to exceed Voyager's resolution, and over the past four years images
from the 10-metre Keck telescope, using its new adaptive optics system and
an infrared camera, have done even better. These observations have shown
that some storm systems on Uranus come and go within days, while others
can persist for years. The fastest winds ever recorded, seen in late 2003,
were up to 420 kilometers per hour.

Magnetic field

Uranus's magnetic field is odd in that it isn't centered on the center of
the planet and that is tilted almost 60° with respect to the axis of rotation.
It is probably generated by motion at relatively shallow depths within Uranus.
(Neptune has a similarly displaced magnetic field, suggesting that this
is not necessarily a result of Uranus's axial tilt.) The magnetosphere
is twisted by the planet's rotation into a long corkscrew shape behind the
planet.

Composition of Uranus

Uranus probably has a solid, rocky core, surrounded by a deep layer of ammonia
and water that has condensed into an icy slush. The outer layer is made
up primarily of liquid hydrogen and helium.